1. The Field of the Invention
The present invention relates generally to x-ray tubes. More particularly, embodiments of the present invention relate to an x-ray tube cooling system that increases the rate of heat transfer from the x-ray tube so as to significantly improve tube performance and at the same time control stress and strain in the x-ray tube structures and thereby extend the operating life of the device.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis and testing. While used in a number of different applications, the basic operation of x-ray tubes is similar. In general, x-rays, or x-ray radiation, are produced when electrons are accelerated, and then impinged upon a material of a particular composition.
Typically, this process is carried out within a vacuum enclosure. Disposed within the evacuated enclosure is an electron generator, or cathode, and a target anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes electrons to be emitted. A high voltage potential is then placed between the anode and the cathode, which causes the emitted electrons accelerate towards a target surface positioned on the anode. Typically, the electrons are xe2x80x9cfocusedxe2x80x9d into an electron beam towards a desired xe2x80x9cfocal spotxe2x80x9d located at the target surface.
During operation of an x-ray tube, the electrons in the beam strike the target surface (or focal track) at a high velocity. The target surface on the target anode is composed of a material having a high atomic number, and a portion of the kinetic energy of the striking electron stream is thus converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as a patient""s body. As is well known, the x-rays can be used for therapeutic treatment, or for x-ray medical diagnostic examination or material analysis procedures.
In addition to stimulating the production of x-rays, the kinetic energy of the striking electron stream also causes a significant amount of heat to be produced in the target anode. As a result, the target anode typically experiences extremely high operating temperatures. At least some of the heat generated in the target anode is absorbed by other structures and components of the x-ray device as well.
A percentage of the electrons that strike the target surface rebound from the surface and then impact other xe2x80x9cnon-targetxe2x80x9d surfaces within the x-ray tube evacuated enclosure. These are often referred to as xe2x80x9csecondaryxe2x80x9d electrons. These secondary electrons retain a significant amount of kinetic energy after rebounding, and when they impact these other non-target surfaces, a significant amount of heat is generated. This heat can ultimately damage the x-ray tube, and shorten its operational life. In particular, the heat produced by secondary electrons, in conjunction with the high temperatures present at the target anode, often reaches levels high enough to damage portions of the x-ray tube structure. For example, the joints and connection points between x-ray tube structures can be weakened when repeatedly subjected to such thermal stresses. Such conditions can shorten the operating life of the tube, affect its operating efficiency, and/or render it inoperable.
The consequences of high operating temperatures and inadequate heat removal in x-ray tubes are not limited solely to destructive structural effects however. For example, even in relatively low-powered x-ray tubes, the window area can become sufficiently hot to boil coolant that is adjacent to the window. The bubbles produced by such boiling may obscure the window of the x-ray tube and thereby compromise the quality of the images produced by the x-ray device. Further, boiling of the coolant can result in the chemical breakdown of the coolant, thereby rendering it ineffective, and necessitating its removal and replacement. Also, the window structure itself can be damaged from the excessive heat; for instance, the weld between the window structure and the evacuated housing can fail.
While the aforementioned problems are cause for concern in all x-ray tubes, these problems become particularly acute in the new generation of high-power x-ray tubes which have relatively higher operating temperatures than the typical devices. In general, high-powered x-ray devices have operating powers that exceed 40 kilowatts (kw).
Attempts have been made to reduce temperatures in x-ray tubes, and thereby minimize thermal stress and strain, through the use of various types of cooling systems. However, previously available x-ray tube cooling systems and cooling media have not been entirely satisfactory in providing effective and efficient cooling. Moreover, the inadequacies of known x-ray tube cooling systems and cooling media are further exacerbated by the increased heat levels that are characteristic of high-powered x-ray tubes.
For example, conventional x-ray tube systems often utilize some type of liquid cooling arrangement. In many of such systems, a volume of a coolant is contained inside the x-ray tube housing so as to facilitate natural convective cooling of x-ray tube components disposed therein, and particularly components that are in relatively close proximity to the target anode. Heat absorbed by the coolant from the x-ray tube components is then conducted out through the walls of the x-ray tube housing and dissipated on the surface of the x-ray tube housing. However, while these types of systems and processes are adequate to cool some relatively low powered x-ray tubes, they may not be adequate to effectively counteract the extremely high heat levels typically produced in high-power x-ray tubes.
As suggested above, the ability of conventional cooling systems to absorb heat from the x-ray device is primarily a function of the type of coolant employed, and the surface area of the x-ray tube housing. Most conventional systems have focused on the use of various coolants to effect the required heat transfer.
Coolants typically employed in conventional cooling systems include dielectric, or electrically non-conductive, fluids such as dielectric oils or the like. One important function of these coolants is to absorb heat from electrical and electronic components, such as the stator, disposed inside the x-ray tube housing. In order to effect heat removal from these components, the coolant is typically placed in direct contact with them. If the coolant were electrically conductive, rather than dielectric, the coolant would quickly short out or otherwise damage the electrical components, thereby rendering the x-ray tube inoperable. Thus, the dielectric feature of the coolants typically employed in conventional x-ray tube cooling systems is critical to the safe and effective operation of the x-ray tube.
While dielectric type coolants thus possess some properties that render them particularly desirable for use in x-ray tube cooling systems, the capacity of such coolants to remove heat from the x-ray tube is inherently limited. As is well known, the capacity of a cooling medium to store thermal energy, or heat, is often expressed in terms of the specific heat of that medium. The specific heat of a given cooling medium is at least partially a function of the chemical properties of that cooling medium. The higher the specific heat of a medium, the greater the ability of that medium to absorb heat.
Thus, the relatively low specific heat (c), typically in the range of about 0.4 to about 0.5 BTU/lb. xc2x0 F., of the cooling media employed in conventional x-ray tube cooling systems have a significant limiting effect on the ability of those media to effect the heat transfer rates that are necessary to ensure the efficient operation and long life of x-ray tubes, and particularly, high-power x-ray tubes. As previously discussed, there are a variety of undesirable consequences when the x-ray tube produces more heat than the coolant can effectively absorb.
The inability of dielectric oils or the like to effect the rates of heat transfer necessary to ensure the efficient operation and long life of x-ray tubes, and particularly, high-power x-ray tubes, is further aggravated by the relatively inefficient manner in which those coolants are employed. In particular, the volume of coolant contained inside the x-ray tube housing is relatively stagnant, and does not circulate throughout the housing. Thus, the cooling effect provided by the coolant is limited primarily to natural convection, a relatively inefficient cooling process, and one that is particularly unsuited to meet the demands of high-power x-ray devices.
Another problem with conventional x-ray tube cooling systems such as those discussed herein concerns the limited volume of coolant available for cooling. A lower volume of fluid affects the heat capacity of the cooling system. Thus, the limited capacity of the coolant employed in conventional x-ray tube cooling systems to absorb heat may limit the system""s efficiency.
In view of the foregoing problems and shortcomings with existing x-ray tube cooling systems, it would be an advancement in the art to provide a cooling system that effectively removes heat from the x-ray tube at a higher rate than is otherwise possible with conventional cooling systems and cooling media. Further, the cooling system should effect sufficient heat removal so as to reduce the amount of thermally-induced mechanical stresses and strain otherwise present within the x-ray tube, and thereby increase the overall operating life of the x-ray tube. Likewise, the cooling system should substantially prevent heat-related damage from occurring in the materials used to fabricate the vacuum enclosure, and should reduce structural damage occurring at joints between the various structural components of the x-ray tube.
The present invention has been developed in response to the current state of the art, and in particular, in response to these and other problems and needs that have not been fully or adequately solved by currently available x-ray tube cooling systems. In general, presently preferred embodiments of the present invention provide an x-ray tube cooling system that effectively and efficiently removes heat from x-ray tube components at a higher rate than is otherwise possible with conventional x-ray tube cooling systems and cooling media. Preferably, embodiments of the x-ray tube cooling system remove sufficient heat from the x-ray tube so as to reduce the occurrence of thermally induced stresses and strain that could otherwise reduce the x-ray tube""s operating efficiency, limit its operating life, and/or render the tube inoperable. Embodiments of the present invention are particularly suitable for use with high-powered x-ray tubes employing a grounded anode configuration.
In a preferred embodiment, the x-ray tube cooling system incorporates a dual coolant configuration. A volume of a first coolant, preferably a dielectric oil or the like, is confined inside the x-ray tube housing in a manner so as to absorb heat from the stator and other components disposed in the housing. Preferably, a pump or the like is employed to circulate the first coolant inside the housing so as to enhance the efficiency of heat absorption by the first coolant. In one alternative embodiment, the first coolant is routed to a heat exchange mechanism, such as a radiator or the like.
Another portion of the dual coolant configuration is a closed coolant circuit that includes a shield structure and a target cooling block, each of which include fluid passageways that are in fluid communication with a coolant pump and radiator, or similar heat exchange mechanism. Preferably, the target cooling block is disposed substantially proximate to the target anode so as to absorb at least some heat therefrom. In a preferred embodiment, at least a portion of the target cooling block is also in contact with the first coolant. Also, in preferred embodiments, the dual coolant configuration includes an accumulator for maintaining a desired level of pressure in the system, and for accommodating volumetric changes in a second coolant due to thermally induced expansion.
In operation, the second coolant, preferably a propylene glycol and water solution or the like, is passed through the radiator by the coolant pump so that heat is removed from the second coolant. Thus cooled, the second coolant then exits the heat exchanger and passes into the fluid passageway of the x-ray tube shield structure, absorbing heat generated in the shield structure by the impact of secondary electrons. After passing through the fluid passageway of the shield structure, the second coolant then enters the fluid passageway defined in the target cooling block and absorbs a portion of the heat dissipated by the first coolant. The second coolant also absorbs heat transmitted to the target cooling block by the target anode. After exiting the fluid passageway of the target cooling block, the second coolant then returns to the coolant pump to repeat the cycle.
The second coolant also serves to remove heat from the first coolant that is disposed within the x-ray tube housing. To maximize this heat transfer, preferred embodiments include means for transferring at least a portion of the heat in the first coolant to the second coolant. This function can be provided by way of a number of different types of heat transfer mechanisms, such as fins, heat sinks, heat pipes, fluid-to-fluid heat exchange devices, and the like.
As the second coolant circulates and absorbs heat from the x-ray tube structures and the first coolant, the temperature of the second coolant, and thus its volume, increases. The accumulator provides a space which serves to accommodate the increase in second coolant volume due to increased temperature. As a result of the increase in second coolant volume, the system pressure increases. The accumulator permits the pressure in the second coolant system to reach a predetermined point, and then maintains the pressure of the second coolant at that point. By maintaining the pressure of the second coolant at a desired level, the accumulator thereby serves to facilitate a relative increase in the boiling point, and thus the heat absorption capacity, of the second coolant.